Polymer fuel cells are expected to play a key role in new energy technologies in the future. A polymer fuel cell using a polymer ion-exchange membrane as an electrolyte is capable of operation at low temperature, and it can be made smaller and lighter. Thus, application thereof for mobiles such as automobiles and portable devices for consumer use has been considered. Fuel cell vehicles carrying polymer fuel cells particularly have drawn social attention as the ultimate ecology cars.
A polymer fuel cell involves the use of a membrane electrode assembly (MEA) comprising catalysts capable of oxidizing a fuel and reducing an oxidant on both surfaces of the ion-exchange membrane and gas diffusion electrodes outsides thereof. Specifically, such structure comprises an ion-exchange membrane of a polymer electrolyte membrane that selectively transports hydrogen ions and electrode catalyst layers mainly composed of carbon powders carrying platinum metal catalysts on both sides of the ion-exchange membrane. Subsequently, a gas diffusion layer that allows a fuel gas to permeate and it conducts electrons is provided on the outer surface of the electrode catalyst layer. In general, a gas diffusion layer is made of a carbon paper or carbon cloth. An electrode catalyst layer and a gas diffusion layer are referred to as an “electrode” in combination.
In the past, for example, a transfer method for integrating a catalyst layer and an electrolyte membrane as disclosed in JP Patent Publication (kokai) No. 2000-90944 (A) has been a principle technique for forming such membrane electrode assembly. In the transfer method, a catalyst mixture in ink or paste state is applied on a substrate, on which the catalyst layer is to be formed, by a method such as sedimentation, printing and spraying and thus the uniform catalyst layer is formed, and subsequently the catalyst layer is subjected to a thermocompression bonding with the electrolyte membrane. In the method, the polymer electrolyte membrane is integrated with the catalyst layer provided on the catalyst-layer-carrying substrate via heating under pressure with the use of a hot press or a hot roller (a hot pressure roller) (hereafter, this technique is referred to as a “hot press technique”).
For example, JP Patent Publication (kokai) No. 10-64574 (A) (1998) discloses techniques involving the use of a hot roller (a hot pressure roller) and a hot press, as shown in FIG. 1 and FIG. 2. FIG. 1 shows the technique involving the use of a hot roller disclosed in JP Patent Publication (kokai) No. 10-64574 (A) (1998). In the technique, firstly, a long polymer electrolyte membrane 1 is integrated with catalyst layers 2 and 3 by subjecting the polymer electrolyte membrane 1 and long films 4 and 5, which are long catalyst-layer-carrying substrates carrying the catalyst layers 2 and 3 respectively and which are placed in both sides of the polymer electrolyte membrane 1, to a thermocompression bonding with the use of a pair of hot pressure rollers 6 which sandwich them. Subsequently, the films 4 and 5 carrying the catalyst layers 2 and 3 are then peeled from the catalyst layers 2 and 3 with the use of a pair of peel rollers 7.
JP Patent Publication (kokai) No. 10-64574 (A) (1998) also discloses a technique for transferring a catalyst layer formed on a catalyst-layer-carrying substrate to a polymer electrolyte membrane using a hot press. FIG. 2 schematically shows a method for transferring a catalyst layer formed on a film to an electrolyte layer using a hot press. As shown in FIG. 2, the electrolyte membrane 10 is sandwiched by films 6 on which the catalyst layers 9 are formed, and the catalyst layers 9 are transferred from the film 6 to the electrolyte membrane 10 by applying a pressure of 5 to 20×106 [Pa] at 80° C. to 150° C. with the use of hot presses 11A and 11B.
In the case of a membrane electrode assembly produced by the hot press technique, however, an electrolyte membrane is not satisfactorily assembled with an electrode catalyst layer, and ion resistance at the boundary between the electrolyte membrane and the electrode catalyst layer is disadvantageously increased. Also, an increased heating temperature or pressure at the time of hot pressing for the purpose of realizing satisfactorily assembled conditions would disadvantageously damage the electrolyte membrane, which would disadvantageously lower the strength of the membrane or the capacity for ion exchange. Further, an increased pressure at the time of hot pressing for the purpose of realizing satisfactorily assembled conditions would consolidate the electrode catalyst layer (i.e., making the electrode catalyst layer nonporous), which would disadvantageously lower gas diffusion in the electrode catalyst layer.
Then, JP Patent Publication (kokai) No. 2002-93424 (A) discloses a method for producing a membrane electrode assembly comprising an assembling step wherein a proton exchange membrane and/or electrode catalyst layer, which previously have been impregnated with a solvent, are pressurized and heated without being soaked in a solvent.